1. Field of the Invention
The present invention relates to a thin-film semiconductor apparatus, a display apparatus using such thin-film semiconductor apparatus, and a method of manufacturing such display apparatus. More particularly, the invention relates to a construction of a gate electrode and a gate wiring of bottom-gate-type thin-film transistors integrally formed as the thin-film semiconductor apparatus. This invention further relates to a display apparatus which employs thus constructed thin-film semiconductor apparatus, and this invention relates to a method of manufacturing a display apparatus employing thus constructed thin-film semiconductor apparatus.
2. Description of the Related Art
A thin-film semiconductor apparatus comprising a number of integrally formed bottom-gate-type thin-film transistors and a display apparatus using such bottom-gate-type thin-film transistors are respectively disclosed in the Japanese Laid-Open Patent Publication No. HEISEI-11-153808/1999, the Japanese Laid-Open Patent Publication No. HEISEI-11-258633/1999, and the Japanese Laid-Open Patent Publication No. HEISEI-11-259016/1999, for example.
Generally, the thin-film semiconductor apparatus such as disclosed in the above identified publications comprises a lower-side wiring having a gate wiring and an auxiliary capacitor wiring formed on an insulating substrate, a gate electrode to be set to the same potential with the lower-side wiring and being patterned simultaneously with the lower-side wiring, a gate insulating film formed on the lower-side wiring and the gate electrode, a semiconductor thin-film formed on the gate insulating film to make a device area of the thin-film transistor TFT, an inter-layer insulating film formed on the semiconductor thin-film, and upper-side wiring having signal wiring which is formed on the inter-layer insulating film and being connected to the thin-film transistor TFT via contact holes.
Structurally, the conventional bottom-gate-type thin-film transistors are composed of a plurality of semiconductor thin-films laminated on a gate electrode via a gate insulating film. The semiconductor thin film is made of polycrystalline silicon crystallized via a laser-beam irradiation process. Inasmuch as the so-called xe2x80x9ca laser-annealing processxe2x80x9d can produce polycrystalline silicon containing satisfactory characteristics via a relatively low temperature, the laser-annealing process itself has become an important processing technique for producing polycrystalline silicon thin-film transistors at a relatively low temperature. However, when a substrate is irradiated with a laser beam, the substrate is momentarily heated to such a high temperature beyond 1000xc2x0 C., and thus, the gate electrode and the gate wiring have to be composed of heat-resistive metal having a high melting temperature. On the other hand, when utilizing such heat-resistive metal, increasing electric resistance of the gate wiring becomes critical problem with reference to a enlarged monitor screen size and finer resolution capability of display devices. As the electric resistance becomes larger, a time constant of the gate wiring also becomes large, thereby gate pulses are delayed due to the enlarged time constant. This delay in turn causes shading on both sides of a displayed image on the monitor screen to degrade display characteristics. To cope with this, specific metal material with lower electric resistance is preferably utilized for the gate wiring. Nevertheless, along with further expansion of the monitor screen size, in order to reduce the electric resistance, it is essential that film thickness of the gate wiring has to be increased.
On the other hand, from the standpoint of production technology, it is known that the thicker the film thickness of the gate wiring is, the worse the step-coverage of the gate insulating film becomes, and this causes inter-layer short-circuit. Concretely, when the film thickness of the gate wiring becomes large, it causes a significant stepwise difference to be generated on the surface of the insulating substrate. Such significant stepwise difference can hardly be leveled off by means of a thin gate insulating film. Once the semiconductor thin film and the signal wiring are formed on the surface of the insulating substrate bearing substantial stepwise difference, short-circuit failure occurs between the lower-side wiring and the upper-side wiring at the portion where the step-coverage is not fully secured. The inter-layer short-circuit has long been a critical problem in order to achieve mass production of large-sized display apparatus having satisfactory quality.
According to the conventional production technology, it was quite difficult to bilaterally satisfy the need for reducing the electric resistance of the gate wiring relative to the expansion of the monitor screen size and the need for reducing the thickness of the gate wiring film being essential for promoting the production yield.
Generally, the gate electrode is composed of layered material identical to the gate wiring and the auxiliary capacitor wiring. When the thickness of the gate wiring is increased, the thickness of the gate electrode is inevitably increased. Increased film thickness of the gate electrode gravely affects crystallization of the semiconductor thin-film by means of the laser-annealing process. When the film thickness of the gate electrode increases, in terms of thermodynamics, a thermal capacity of the metal to act as a thermal absorptive portion against silicon layers increases during the laser-beam irradiation process to make it difficult to efficiently heat up the silicon layer. When excess energy is fed to the silicon layer in order to compensate for the amount of heat released from the silicon layer, it in turn causes defect in the crystallized silicon thin film, thus also raising a problem.
In order to fully solve the above problems, the invention presents a first means by way of the following. Concretely, the invention provides a novel thin-film semiconductor apparatus comprising a plurality of thin-film transistors integrally formed on an insulating substrate. The novel thin-film semiconductor apparatus characteristically comprises lower-side wiring formed on the insulating substrate, a gate electrode to be set to the same potential with the lower-side wiring and being patterned simultaneously with the lower-side wiring, an insulating film formed over the lower-side wiring and the gate electrode, a semiconductor thin film formed on the insulating film to make a device area of the thin-film transistor, an inter-layer film formed on the semiconductor thin-film, and an upper-side wiring formed on the inter-layer film and being connected to the thin-film transistors via contact holes. Characteristically, the gate electrode has such a thickness being less than that of the lower-side wiring.
Concretely, the above-referred gate electrode and the lower-side wiring are respectively composed of a layered structure. The number of layers for composing the gate electrode is less than the number of layers for composing the lower-side wiring. More concretely, the gate electrode is composed of a single layered structure solely consisting of a surface layer, whereas the lower-side wiring is composed of multi-layered structure comprising an inner layer being formed below the surface layer. In this case, each of the inner layers is made from a metal having lower electric resistance value than that of the surface layer, whereas the surface layer is made from such metal having higher melting temperature than that of the inner layer. For example, each of the inner layer is made from aluminum-based metal. The surface layer is made from any metal selected from molybdenum, tantalum, tungsten, or chromium. Preferably, the semiconductor thin-film comprises polycrystalline silicon crystallized by a laserbeam irradiation process.
Further, in order to more fully solve the above-referred problems, the invention presents a second means. Concretely, the invention further provides a novel thin-film semiconductor apparatus comprising a plurality of thin-film transistors integrally formed on an insulating substrate. The novel thin-film semiconductor apparatus characteristically comprises a lower-side wiring formed on an insulating substrate, a gate electrode connected to the lower-side wiring, an insulating film formed on the lower-side wiring and the gate electrode, a semiconductor thin-film formed on the insulating film to form a device area of the thin-film transistor, an inter-layer film formed on the semiconductor thin-film, and an upper-side wiring formed on the inter-layer film and connected to the thin-film transistors via contact holes.
The above-referred lower-side wiring comprises a multi-layered structure in which at least a second wiring layer is superposed on a first wiring layer. Further, width of the second wiring layer is arranged to be greater than the width of the first wiring layer to enable the second wiring layer to fully cover the first wiring layer. Further a size from an end of the first wiring layer to an end of the second wiring layer projecting to the outer portion in the width direction is arranged to be greater than the thickness of the second wiring layer.
Preferably, the first wiring layer is made from a metal having a lower electric resistance than that of the second wiring layer and the second wiring layer is made from a metal having higher melting temperature than that of the first wiring metal. For example, the first wiring layer is composed of aluminum-based metal, whereas the second wiring layer is composed of any metal selected from molybdenum, tantalum, tungsten, or chromium. In actual forms for carrying out the invention, the above-referred first wiring layer comprises multi-layered structure in which at least the second wiring layer is superposed on the first wiring layer, whereas the above-referred gate electrode comprises a single-layer solely comprising the second wiring layer. The above-referred semiconductor thin film comprises polycrystalline silicon crystallized by the laser-beam irradiation process.
According to the first means of the invention, thickness of the gate electrode of the bottom-gate-type thin-film transistor is arranged to be thinner than that of the lower-side wiring including the gate wiring and the auxiliary capacitor wiring. The thinner the gate electrode is, the less the amount of heat being released via laser-beam irradiation becomes. Because of this, the laser-beam energy can effectively be utilized for crystallization of the semiconductor thin film. On the other hand, inasmuch as sufficient thickness can be provided for the lower-side wiring including the gate wiring and the auxiliary capacitor wiring, the electric resistance can be minimized to make it possible to prevent the time constant from being increased even when the size of the monitor screen becomes larger.
According to the second means of the invention, the lower-side wiring including the gate wiring and the auxiliary capacitor wiring comprises a multi-layered structure in which the second wiring layer having surpassing heat-resistant property is superposed on the first wiring layer having relatively low electric resistance. Width of the second wiring layer (surface layer) is arranged to be wider than that of the first wiring layer (inner layer) to enable the surface layer to fully cover the inner layer. Further, dimension ranging from an end of the inner layer to an end of the surface layer projecting to the outer portion in the width direction is arranged to be greater than the thickness of the surface layer. By forming the stepwise cross-sectional structure by providing the inner-layer below the surface-layer, stepwise difference of the lower-side wiring is reduced to eventually make it possible to prevent inter-layer short-circuit. By suppressing occurrence of such inter-layer short-circuit, production yield can be promoted. Further, by way of utilizing metal material containing low electric resistance for composing the inner layer, increase of electric resistance of the entire lower-side wiring can be suppressed.